1. Field of the Invention
The present invention relates to an optical waveguide device, more particularly, to an optical waveguide device such as modulators, switches, distributors, or the like, used in an optical communication apparatus.
2. Description of the Related Art
A typical optical waveguide device used in an optical switch, an optical modulator, or the like, is constituted such that electric field is applied to an optical waveguide formed on a surface of a substrate consisting of electrooptical crystal such as lithium niobate (LiNbO.sub.3), or the like, so as to change a refractive index and thus switching or phase modulation of a light signal traveling through the optical waveguide can be carried out.
As an example of the optical waveguide device, a Mach-Zehnder type modulator is well known. A modulator of this type includes at least an optical waveguide formed on a surface of a Z-cut LiNbO.sub.3 substrate and a pair of asymmetrical electrodes formed on corresponding regions above the optical waveguide. In such a structure, charges collected on the surface of the LiNbO.sub.3 substrate by pyroelectric effect act on the asymmetrical structure of the electrodes and thus a disadvantage occurs in that a distribution of charges is made uneven and an adverse influence is exerted on characteristics of the device.
Also, since a distribution of resistance, dielectric constant and capacitance of each portion of the structure or partial fine structure is delicately changed in accordance with the process conditions, the electrical equivalent circuit accordingly becomes complicated. This results in a problem in that a direct current (DC) component of an electric signal applied across the electrodes greatly changes an application manner of the electric field with a long-term time constant and the optical response is also accordingly changed. This phenomenon is called a DC drift.
To cope with the problem, various measures have been heretofore taken. However, a satisfactory and effective technique of solving the problem has not yet been proposed.
Note, problems in the prior art will be explained in detail later in contrast with the preferred embodiments of the present invention.